Heating apparatus



March 14, 1950 w. M. HALL ET AL HEATING APPARATUS Filed Jan. 14, 1947M'VENTO/FS w/z mm M mu F/P/TZ A. 6170.55 5/ J in A 27 Patented Mar. 14,1950 HEATING APPARATUS William M. Hall, Lexington, and Fritz A. Gross,

Weston, Mass, assignors to Raytheon Manufacturing Company, Newton, Mass,a corporation of Delaware Application January 14, 1947, Serial No.721,948

This invention relates to heating apparatus, and more particularly to anoven-type microwave heating device.

An object of this invention is to provide an improved door structure formicrowave heaters of the oven type.

Another object is to provide a convenient means of access to metallicenclosures or cavities which are adapted to be supplied with microwaveenergy, which means of access will not affect or interfere with the wavecharacteristics of the cavity.

A further object is to provide a means of access to a metallicradio-frequency cavity, which will not permit any appreciable escape orleakage of microwave energy from the interior of the cavity. Theforegoing and other objects of the invention will be best understoodfrom the following description of an exemplification thereof, referencebeing had to the accompanying drawing, wherein:

Fig. 1 is a front view of an oven according to this invention;

Fig. 2 is a vertical section taken along line 22 of Fig. 1; and

. Fig. 3 is a partial horizontal section, on an enlarged scale, takenalong line 3-3 of Fig. 1.

In oven-type microwave cooking devices, the problem arises of providingadequate shielding of the door opening to prevent escape ofradiofrequency energy. As the field intensity inside the oven is veryhigh, this may be an important problem, both to minimize radio-frequencyinterierence and to prevent radio-frequency burns if a person happens tocome into contact with the edge of the door. Any feasible type or springfinger contact, between the door and the oven, which would provideadequate contact, would be extremely difficult to keep clean. Similarproblems would be presented by a conventional type of radio-frequencychoke structure.

It has been found that a planar contact type closure, of such size thatthe metallic contact area between the door and the oven is approximatelya quarter-wavelength long, is very effectiva'giving substantiallycomplete shielding yet being free from sparking. Such aclosure is withinthe scope of'this invention.

An improved closure contact structure has been devised, however, whichgives 'bettershielding than the single quarter-wavelength overlapping,does not require a particularly good contact surface, and has an easysurfaceyto keep clean. This improved contact structure, will bedescribed in amore detailed manner below. m ght 8 Claims. (Cl. 219-47)be termed a two-stage choke, and consistsof two metallic contactsurfaces or areas eachapproximately 2, quarter-Wavelength long,separated by a groove approximately a quarter-wavelength deep, saidcontact surfaces and groove being integral with the closure.

Now referring to the drawing, a hollow rec.- tangular prismoidalenclosure or cavity I is made of a suitable metal and has rather thinwalls as shown; enclosure I is adapted to serve as the oven of thecooking apparatus. Oven I is, adapted to have a container 2 of foodplaced therein, said container resting on the bottom wall of the ovenWhile the food therein is being cooked. In order to allow access to theinterior of the oven for placing the food therein and for removing thefood therefrom, a rectangular aperture 3 isprovided in the front wall ofthe oven l near the lower end of said wall, this aperture being closableby means of a pivotally-supported metal door 4, to be described more indetail hereinafter. When dOOr 4 is closed, theenclosure I is entirelyclosed, except for the opening H for the exciting means to be describedhereinafter.

Numeral 5 generally designates an electrondischarge device of themagnetron type, which includes, for example, an evacuated envelope '6,made of highly conductive material, such as copper, and provided with aplurality of inwardly-,

directed, radially-disposed anode vanes '1. The arrangement is such thateach pair of adjacent anode vanes 1 forms, together with that portionofthe envelope lying therebetween, a cavity resonator whose naturalfrequency is, as is well known to those skilled in the art, a functionof the geometry of the physical elements making up the same, For thepurposes of the present invention it is desirable thatthe dimensions ofeach such cavity resonator be such that the wavelength of the electricaloscillations adapted to be generated therein has a predetermined value,for example, of the order of ten centimeters. Wave+ lengths of thisorder lie in the microwave region of the frequency spectrum. However,this invention is equally applicable to radio-frequency energy of longeror shorter-wavelengths, such wavelengths lying within or without themicro wave region.

Centrally located in envelope 5 is a highly electron-emissive cathodemember 8, for example, of'th-e well-known alkaline-earth metal oxidetype, said cathode member being provided with conventional means (notshown) for raising the temperature thereof to level sufiicient forthermionic emission.

The electron-discharge device 5 is completed by magnetic means (notshown) for establishing a magnetic field in a direction transversely ofthe electron path between the cathode and anode members thereof.

Magnetron 5 is energized from any suitable source (not shown) and Whenso energized delivers microwave energy or high-frequency electromagneticwaves to a coaxial transmission line 9 the inner conductor ID of whichis coupled to oscillator 5 by a loop II and the outer conductor of whichis connected to envelope 6. The inner conductor ID of line 9 extendsdirectly into the interior of oven I through a suitable opening I2provided in the rear wall thereof, while the outer conductor of saidline is connected to the rear wall of the oven I by a suitable fasteningmeans I3. Opening I2 is preferably centered with respect to the verticalside walls of the oven, as shown in Fig. 1, and is preferably somewhatabove th horizontal midplane of the oven, as shown in Figs. 1 and 2.

The front wall Id of the oven, as shown in Figs. 1 and 3, extends anappreciable distance beyond the oven I at the sides and bottom thereof,to provide side and bottom outwardly-projecting flanges at the front ofthe oven. A hinge structure I4, fixed to the bottom flange of the oven,pivotally supports door 4 on the oven. Door 4 is substantiallyrectangular in configuration and has a planar or flat central portion4a, a flat pew ripheral or outer flange portion 4?), and aninwardly-opening outwardly-extending rib portion 40 between portions 4aand 4b. Portions 4a, 4b, and 4c are all substantially rectangular inconfiguration. The internal depth a of the inwardly-opening groove whichforms rib portion 40, measured from the inner surface of the door, has avalue such that it is approximately equal to one-fourth of thewavelength of the waves generated by oscillator 5, and this internaldepth is constant throughout all four sides of the rib portion. Thetotal area of the inner face of door 4 is substantially larger than thearea of rectangular aperture 3, and door 4 is so positioned that thecenter of the doors inner face area coincides with the center ofaperture 3.

The central portion 4a of door 4 has an area somewhat larger thanrectangular aperture 3, as shown in Fig. 1, and the distance, on allfour sides of aperture 3, from the edge of said aperture to the nearestor inner edge of the corresponding groove forming rib portion 40, ismade equal to a and is therefore approximately one-quarter wavelength ofthe waves generated by oscillator 5. This aforesaid distance is ofcourse measured when the door is in place on oven I.

The distance, on all four sides of door 4, from the extreme outer edgeof flange portion 4b to the nearest or outer edge of the correspondinggroove forming rib portion 40, is also made equal to a and is thereforealso approximatel onequarter wavelength.

The inner planar surface of the door 4 is made reasonably smooth oruniform, and in the closed position of the door, which is the onlyposition thereof illustrated in the drawing, the inner planar surface ofsaid door is in firm metallic contact with the front wall la ofenclosure I in a region thereof adjacent to and immediately surroundingall sides of aperture 3, as is apparent from Figs. 2 and 3. The Side andbottom outwardly-projecting flanges of wall Ia assist in achieving suchcontact.

Since the distances a are all equal to a quarterwavelength, two surfacesor areas of contact be tween the door and the oven are provided, thesesurfaces or areas each being approximately a quarter-wavelength long (orwide), said contact surfaces being separated by a groove in portion 40which is approximately a quarter-wavelength deep. One of these areasextends between the edge of aperture 3 and the inner edge of the groovein door portion 4c, and the other of these contact areas extends betweenthe outer edge of said groove and the outer edge of door portion 4b. Theabove-described closure contact structure may for convenience be termeda two-stage choke with a groove between the two stages.

A handle I5 is attached to door 4 to enable manual opening and closingthereof.

It is desirable to prevent leakage of microwave energy from the interiorof cavity or enclosure I, when door 4 is closed and oscillator 5 isenergized to produce high-frequency electromagnetic waves or microwaveenergy. Microwave energy is fed by coaxial line 9 and exciting probe orrod ID, from magnetron oscillator 5 to the interior of oven or cavity I.Leakage of microwave energy from the interior of cavity I wouldordinarily take place through aperture 3, unless the door structure weredevised to substantially prevent such leakage. In this invention, thestructure of door 4, and its contact with enclosure I, are designed toshield the aperture 3 to prevent escape of radiofrequency energytherethrough.

Due to the fact that the inner planar surface of the door is onlyreasonably smooth or uniform, good but not perfect contact between thedoor and the outer surface of the enclosure is provided throughout theinnermost or first contact surface or area. Therefore, this contact areaacts as a quarter-wavelength choke with a minute amount of insulation,presenting a high impedance at the edges of the aperture 3 andpreventing any appreciable leakage of microwave energy through or overthe edges of said aperture to the exterior of cavity I.

The groove in door portion 4c, which groove is a quarter-wavelengthdeep, provides a discontinuity at the edge or end of the first contactarea, above referred to. Adjoining or surrounding the groove at itsouter edge is a second contact area, as above described. This secondarea is also approximately a quarter-wavelength long and functions as aquarter-wave choke as does the first contact area, so that said secondarea functions as the second stage of the two-stage choke. The twocontact areas or quarter-wave chokes are effectively separated by thediscontinuity provided by the groove, and effectively function incombination as a two-stage radiofrequency choke the two stages of whichare in series with each other.

It has been found that such a two-stage choke provides exceptionallycomplete shielding, does not require particularly good contact surfaces,and has an easy surface to keep clean. If there is metallic contactbetween the door and the enclosure, th radio-frequency choke functionsas desired, but if there is no contact the structure functions as alow-impedance quarter-wave choke.

As stated above, a contact type closure, in which the contact isapproximately a quarterwavelength long, has been found to be veryeffective also. In this case, the structure functions as a singlequarter-wave choke, presenting a high impedance at the edges of theaperture. giving very complete shielding yet being free from sparking.

Of course, it is to be understood that this invention is not limited tothe particular details as described above, as many equivalents willsuggest themselves to those skilled in the art. It is accordinglydesired that the appended claims be given a broad interpretationcommensurate with the scope of this invention within the art.

What is claimed is:

l. High-frequency apparatus, comprising a metallic enclosure, means forsupplying highfrequency electromagnetic waves of a predeterminedWavelength to the interior of said enclosure, said enclosure having anaccess aperture therein, and a metallic door for closing said aperture,said door, upon closure thereof, being in direct metallic contact withsaid enclosure outside of said aperture over a planar contact area Whosewidth is substantially one-quarter of said wavelength.

2. High-frequency apparatus, comprising a metallic enclosure, means forsupplying highfrequency electromagnetic Waves of a predeterminedwavelength to the interior of said enclosure, said enclosure having anaccess aperture therein, and a metallic door for closing said aperture,said door being mounted to allow arcuate motion thereof and upon closurebeing constructed to be in direct metallic contact with said enclosureover a contact area entirely surrounding said aperture, said area havinga width substantially one-quarter of said wavelength.

3. High-frequency apparatus, comprising a metallic enclosure, means forsupplying highfrequency electromagnetic waves of a predeterminedwavelength to the interior of said enclosure, said enclosure having anaccess aperture therein, and a metallic door for closing said aperture,said door, upon closure thereof, being in direct metallic contact withsaid enclosure outside of said aperture over a plurality of spacedcontact areas whose widths are each substantially one-quarter of saidwavelength, and the distance between said areas being substantially lessthan a quarter wave length.

4. High-frequency apparatus, comprising a metallic enclosure, means forsupplying high-frequency electromagnetic waves of a predeterminedwavelength to the interior of said enclosure, said enclosure having anaccess aperture therein, and a metallic door for closing said aperture,said door, upon closure thereof, being in direct metallic contact withsaid enclosure over a pair of spaced planar contact areas each entirelysurrounding said aperture, each of said areas having a widthsubstantially one-quarter of said wavelength.

5. High-frequency apparatus, comprising a metallic enclosure, means forsupplying high-frequency electromagnetic waves of a predeterminedwavelength to the interior of said enclosure, said enclosure having anaccess aperture therein, and a metallic door for closing said aperture,said door, upon closure thereof, being in direct metallic contact withsaid enclosure outside of said aperture over a pair of spaced contactareas whose widths are each substantially one-quarter of saidwavelength, said areas being spaced from each other by a groove whosedepth is substantially one-quarter of said wavelength.

6. High-frequency apparatus, comprising a metallic enclosure, means forsupplying highfrequency electromagnetic waves of a predeterminedwavelength to the interior of said enclosure, said enclosure having anaccess aperture therein, and a metallic door for closin said aperture,said door, upon closure thereof, being in direct metallic contact withsaid enclosure outside of said aperture over a pair of spaced planarcontact areas whose widths are each substan tially one-quarter of saidwavelength, said areas being spaced from each other by a groove in saiddoor whose depth is substantially one-quarter of said wavelength.

7. Heating apparatus including an oven, comprising a metallic enclosure,means for supplying high-frequency electromagnetic waves of apredetermined wavelength to the interior of said enclosure, saidenclosure having an access aperture therein, and a metallic door forclosing said aperture, said door, upon closure thereof, being in directmetallic contact with said enclosure over a pair of spaced contact areaseach entirely surrounding said aperture, each of said areas having awidth substantially one-quarter of said wavelength, said areas beingspaced from each other throughout their extents by a groove whose depthis substantially one-quarter of said wavelength.

8. High-frequency apparatus, comprising a metallic enclosure, means forsupplying highfrequency electromagnetic waves of a predeterminedwavelength to the interior of said enclosure, said enclosure having anaccess aperture therein, and a metallic door for closing said aperture,said door, upon closure thereof, being in direct metallic contact withsaid enclosure over a pair of spaced planar contact areas each entirelysurrounding said aperture, each of said areas having a widthsubstantially one-quarter of said wavelength, said areas being spacedfrom each other throughout their extents by a groove in said door whosedepth is substantially onequarter of said wavelength, and Whose Width issubstantially less than a quarter wave length.

WILLIAM M. HALL. FRITZ A. GROSS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,226,479 Pupp Dec. 24, 19402,364,526 Hansell Dec. 5, 1944 2,370,161 Hansen Feb. 27, 1945 2,407,318Mieher et al. Sept. 10, 1946 2,408,295 Cossin Sept. 24, 1946 2,415,962Okress Feb. 18, 1947 2,439,388 Hansen Apr. 13, 1948

